You can use the Regex.Split method, something like this:
Regex regex = new Regex(@"\bis\b");
string[] substrings = regex.Split("This is a sentence");
foreach (string match in substrings)
{
Console.WriteLine("'{0}'", match);
}
Edit: This satisfies the example you gave. Note that an ordinary String.Split will also split on the "is" at the end of the word "This", hence why I used the Regex method and included the word boundaries around the "is". Note, however, that if you just wrote this example in error, then String.Split will probably suffice.